This is an on-going problem with CB V15. It has been reported by more than one person. It happens to me so often, that I don't even think about it anymore. I just restart CB (sometimes I have to actually kill the application that is left running when CB shuts down due to this) and keep going. Nothing helps (I have rebuilt lists and compacted and verified many times). I am hoping (and have been doing so for a long time) that the (apparently mythical) V16 upgrade will have this resolved. Maybe I will find out one day...